Electrostatically charged image development type copiers and printers are gaining popularity because of widespread office automation. With this background, demand is growing for high grade or sharp, stable copied or printed images. General formulations for carriers in electrostatically charged image developing copiers and printers are shown in Table 1. One of the main factors for improving the sharpness and stability of a copy image is a surface coating resin of the carrier. An electrostatically charged image developing copier or printer feeds a toner to an electrostatically charged image on a latent image carrier to obtain a visible image, then transfers the resulting toner image to a plain paper or an OHP film, and fixes the transferred image. Currently, developers comprising toner particles electrostatically joined to carrier particles of iron powder or carrier particles of ferrite consisting essentially of nickel or cobalt are in wide use as means of supplying toner to the electrostatically charged image on the latent image carrier to obtain the visible image. For the surface of these carriers, fluoro-acrylic graft polymer, cellulose butyl acetate, or silicone resin is used as a coating material to impart charge control properties and anti-spent toner effect (spent toner phenomenon is a phenomenon in which the toner adhering to the surface of the carrier is firmly bonded to the carrier electrostatically, physically or chemically, and thus the toner is not transferred to the electrostatically charged image on the latent image carrier). The coating material proves satisfactory in a copy image initially formed. During its long-term use, however, toner particles that have triboelectrically adhered to carrier particles become difficult to separate from the carrier owing to the accumulation of electrostatic charges between the toner and the carrier. Alternatively, the adhering toner particles are fusion bonded to the carrier surface because of heat of friction, so that the replenishing toner is not charged. As a result, replenishment of toner to the electrostatically charged image on the latent image carrier is insufficient, causing image deterioration, namely, short life of the developer.
TABLE 1 ______________________________________ (% by weight) Core substance Surface coating agent ______________________________________ Iron oxide powder Iron oxide powder 1-30 type carrier 70-99 Ferrite type Ferrite powder 1-30 carrier 70-99 ______________________________________
The present invention has been accomplished in the light of the aforementioned problems. The object of this invention is to provide a coated carrier which is capable for achieving a higher grade copy image, that is, a sharp image, and a long life of the developer, in an electrostatically charged image development type copier or printer.